Supporting 8.1 (Remove deprecated interface) + Update and fix phpunit test

[dannyvw]

@Nyholm @sroze do you have any chance to look at this and maybe merge or create a new release?

[joel-loycom]

This is also blocking us, can it be merged?

[dzianis-hrynko]

@dannyvw @joel-loycom you can use https://packagist.org/packages/autoprotect-group/messenger-enqueue-transport it's a fork that supports 8.1

[qkdreyer]

Would love to see this getting released too

[qkdreyer]

@Nyholm @sroze bump 😞